When a plastic branch tube 1 as shown in FIG. 7 is integrally molded by conventional injection molding, slide molds 4 which are provided to an elevation mold 3 so that they can be moved back and forth are used. That is, the hollow portion of the branch tube 1 is formed by getting the tips of the three slide molds 4 butted against each other, and the branch tube 1 shown in FIG. 7 is produced by withdrawing the slide molds 4 after finishing injection.
However, in the conventional process in which the slide molds are used, the slide molds must be withdrawn after finishing injection. Accordingly, a branch tube 1 which, for example, has a curved hollow portion as shown in FIG. 6 cannot be molded. Moreover, screw portions la cannot be integrally molded, and there is no other way but to form them by another step after molding.